The invention disclosed herein pertains to a magnetic cylinder structure and a method of fabricating the cylinder.
Magnetic cylinders are used for holding thin flexible magnetizable etched plates, such as are used in printing presses or die cutting presses. The type of magnetic cylinder most commonly used at the present time is based on Hotop et al. U.S. Pat. No. 3,097,598. This type of magnetic cylinder comprises a series of uniformly spaced apart coaxial centrally apertured ferrous metal disks, called magnetic poles, having nonmagnetic spacer rings between them. The spaces defined between the poles are occupied by a circular array of permanent bar magnets. Construction of the cylinder involves making subassemblies by setting a nonmagnetic spacer ring on a pole disk in a jig and placing the magnets on the pole disk. These subassemblies are then stacked on a mandrel or the like and finally clamped together to form a cylinder. In a magnetic cylinder design that is currently widely used the inside diameters of the spacers and the pole disks are nominally but not exactly the same. There is, however, sufficient diametral clearance for the spacers and pole disks to make a slip fit onto the mandrel with little force being applied. Because, in practice, it is impossible to make the two internal diameters the same size, both inside diameters are made oversize by, perhaps, two thousandths of an inch to assure that the magnetic pole disks and the spacers will fit on the mandrel. If it were possible to make both diameters exactly equal, then the cylinder could be press fit on the mandrel. To compensate for the dimensional tolerance and hopefully prevent slight radial shifting of the disk and magnet subassemblies, epoxy resin is applied to them to fix them against radial shift and to hold the magnets in place. The periphery of the cylinder is usually ground and polished to a very smooth finish. Sometimes the cylinders are chromium plated. Even though the pole disks and spacers are bonded with epoxy resin, they sometimes yield radially when in use so concentricity is lost. The periphery of the cylinder takes on a corduroy appearance and the die plate deforms and can no longer cut elements out of a sheet properly. This is the harmful consequence of having needed relatively large inside diameter tolerances.
Assembling known types of magnetic cylinders is tedious and must be done with considerable care. The spacer and pole must be maintained concentric during bonding in which case specialized jigs may have to be used for performing the bonding operation. In prior art magnetic cylinder designs in current use wherein spacer rings are bonded to the poles with an adhesive such as an epoxy resin the bond is not stable and experience has shown that the parts have a tendency to shift or move in use, that is, when they are rotating at high speed or under high pressure in a printing or die cutting press. Radial slipping by a pole of as little as one ten-thousandth of an inch can make a die unusable for cutting.
A basic objective in magnetic cylinder design is to have the magnetic field strength maximized at the surface of the cylinder for exerting the strongest magnetic attraction on the flexible magnetizable printing or die cutting plate that is wrapped around the magnetic cylinder. The internal circumference of the bendable die plates is substantially equal to the external circumference of the magnetic cylinder in which case the edges of the plate butt or nearly butt against each other. Where the diameter of the magnetic cylinder is relatively small, such as about 3.5 inches or less, the die plate must be formed into a similar diameter such that high internal bending stresses are developed which tend to restore the stiff thin die plate to a flat condition. This causes the edges of the flexible plate to tend to peel away from the periphery of the cylinder. To mitigate this problem, manufacturers have tended to use magnets throughout the magnetic cylinder which have the highest available strength to assure that the magnetic attraction is strong enough to prevent any separation of the flexible plate from the magnetic cylinder in the region of the end edges of the flexible plates. The cost of magnets increases at an exponential rate with increased strength since the stronger magnets are composed of rare and sophisticated materials as compared to the weaker magnets.